1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst to be provided in an exhaust pipe of an internal combustion engine. More specifically, the present invention relates to an exhaust gas purifying catalyst of wall flow type for use in an SCR type purification device.
2. Description of the Related Art
Harmful components such as particulate matter (PM), hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are contained in exhaust gas discharged from internal combustion engines such as automobile engines. Among the abovementioned harmful components, NOx is contained in the exhaust gas discharged from diesel engines, and a selective catalytic reduction (SCR) type purification device capable of selectively purifying this NOx is installed in an exhaust pipe of an internal combustion engine.
The SCR type purification device is provided with a reducing agent supply unit and an exhaust gas purifying catalyst. After the reducing agent (for example, urea) has been supplied to the exhaust gas discharged from the internal combustion engine, the exhaust gas mixed with the reducing agent is passed through the exhaust gas purifying catalyst. A catalyst layer containing an SCR catalyst body is formed inside such an exhaust gas purifying catalyst, and the reducing agent adsorbed on the SCR catalyst body is reacted with NOx in the exhaust gas by the catalytic action of the SCR catalyst body, thereby purifying the NOx in the exhaust gas. For example, Japanese Patent No. 5787031 discloses a technique relating to such an SCR type purification device.
An exhaust gas purifying catalyst used in the SCR type purification device described above is exemplified by an exhaust gas purifying catalyst of wall flow type such as described in Japanese Patent Application Publication No. 2009-82915. The exhaust gas purifying catalyst of wall flow type includes a base of a wall flow structure having inlet-side cells (inlet-side flow paths) where only an end portion on an exhaust gas inflow side is open, outlet-side cells (outlet-side flow paths) where only an end portion on an exhaust gas outflow side is open, and porous partition walls (rib walls) partitioning these cells, and a catalyst layer including an SCR catalyst body is formed in the partition walls of the base.
In such an exhaust gas purifying catalyst of wall flow type, the exhaust gas flowing into the inlet-side cells passes through the porous partition walls and comes into contact with the catalyst layer formed in the partition walls, then flows out to the outlet-side cells and is discharged to the outside of the exhaust gas purifying catalyst. As a result, the exhaust gas mixed with the reducing agent contacts the SCR catalyst body in the catalyst layer, and NOx is purified by the abovementioned SCR reaction.